1. Field.
This invention relates to electrical apparatus for generating high-energy neutrons. It relates in particular to apparatus for generating 14-Mev neutrons which apart from power supply is sufficiently compact to be used in a well-bore for well-logging, and which will deliver a high output of neutrons for extended periods when electrially energized.
2. Prior Art.
Much of the relevant prior art has been reviewed in the applicant's co-pending application, "Neutron Generator Apparatus and Method", Ser. No. 452,080, filed Mar. 18, 1974, which is incorporated herein by reference.
In that review, attention is focused on the methods and designs which have been evolved to provide a long-lived, high-intensity source of 14-Mev neutrons for the treatment of cancer, said neutrons being generated by the T(d,n)He.sup.4 reaction, which is well known to the skilled artisan as the D-T reaction. In the problem of neutron well-logging, D-T neutrons are produced in a compact source which can operate down-hole in order to assess the properties of the materials surrounding the well-bore. Although the applications are widely different, the problems which limit the useful output of the neutron source, so far as intensity and lifetime are concerned, are basically similar. They fall under the two headings of thermal effects and dilution effects.
Targets for the production of D-T neutrons commonly consist of a chemical compound of tritium with a substance which forms a strong hydride bond such as zirconium, titanium, or scandium. These compounds are stable only over a limited temperature range. For titanium, for example, the compound dissociates at temperatures in excess of 200.degree. C. It is necessary therefore to limit the temperature attained by the target. When the target is bombarded by deuterons to produce neutrons, the bombarded areas are often strongly heated. Hence, thermal effects are critical in determining the neutron output and useful lifetime of a target.
Many methods have been devised to control and to improve the thermal characteristics of the target. For example, it is common to support the target on a base of high thermal conductivity, such as a thin sheet of copper, and to cool the back of the copper base efficiently. And in the above-cited co-pending application, a novel, rotating target is described which deals effectively with the thermal problem by effectively spreading the beam over an extended area of target by rapidly rotating the target under the beam, successively exposing different areas of the target to the beam. Such a rotating target is inapplicable to the down-hole situation, where the diameter of the apparatus is constrained to be less than about 1.5 inches in diameter.
Another method of dealing with thermal effects and the less of tritium from the target resulting therefrom is to accelerate a mixture of tritium and deuterium ions, so that the target is continuously replenished by ions from the beam itself. While this method is effective in prolonging the life of the target, it is inherently much less efficient in the number of neutrons per second produced per watt of power dissipated in the target, compared to a system in which deuterium ions impinge on a tritiated target, and is limited in ouput by the electrical power available down-hole to neutron intensities in the range of 10.sup.8 per second. For the same power dissipation, with a deuterium ion beam incident on a tritiated target, the neutron output obtainable is estimated to be at least 10.sup.9 per second, and this is the level of performance which is being sought.
In addition to thermal effects, which cause progressive deterioration of a tritiated target, we must deal with what we have called "dilution" effects. The term "dilution" refers to the fact that as a tritiated target is bombarded by energetic deuterium ions, some of those ions displace atoms of tritium from the target, which then leave the target permanently. The resulting gradual depletion of the tritium content of the target causes a gradual reduction of the neutron yield of the target.
To clarify the dilution effect, it is necessary to take note of the fact that the beams of deuterium ions which are produced by conventional ion sources consist both of atomic and molecular species, in relative proportions which vary from one source to another, and are dependent on the operation conditions of a given source. Usually the dominant species are atomic (D.sup.+) and diatomic (D.sub.2.sup.+), but ions of several more massive species may be present in minor quantities.
The distinction between atomic and molecular species is important because of associated differences in dilution effect. In particular, it is useful to distinguish between the effects of ions of a given species on neutron production by ions of the same species (self-dilution effect), and on those of a different species (cross-dilution effect).
In the work of J. H. Ormrod (Canadian Journal of Physics, 52, 1971 (1974)), it is shown with particular clarity that the dilution takes place primarily in a thin layer of the target at the end of the range of the projectile ions -- that is, in the layer in which the projectiles are implanted. Since tritium is removed primarily from those layers in which the projectile ions are implanted, then the magnitude of the self-dilution effect is determined primarily by the magnitude of the probablility or cross section for production of a D-T neutron by a deuterium ion at or near the end of its range. This probability or cross section is very small, and therefore the self-dilution effect is very small. The situation is quite different, however, for the cross-dilution effect.
Let us consider the simplest, but practically the most important case of cross-dilution -- namely, that involving only the atomic and the diatomic ions of deuterium. And let us assume that the ions impinge on the target after falling through a potential difference of 200 kev. The atomic ions will penetrate the target to a depth corresponding to the range of an atomic, 200 kev projectile of mass 2. The diatomic, mass-4 ions break up on entering the target into two atomic ions which share the energy of the diatomic ion equally, and thus penetrate the target with the energy of two, separate, 100 kev ions of mass 2. These ions will be implanted in a correspondingly shallower layer of the target. It is this shallower layer of the target which is depleted of its tritium in due course. But is so happens that the layer of the target which is depleted of tritium by ions which were originally diatomic is a layer traversed by atomic ions at an energy for which the neutron yield of the D-T reaction is near its maximum value. Thus, cross-dilution has a major effect on the neutron yield of a tritiated target bombarded simultaneously by atomic and diatomic ions which have fallen through the same potential difference.
In the prior patent application referenced above, a method of eliminating cross-dilution is disclosed without loss of ions or the associated neutron production. In that method, the deuterium ions are passed through a magnetic field of fixed direction perpendicular to the axis of the ion beam, causing the latter to be dispersed into a plurality of beams, each of a single mass species, in such manner that each species impinges a separate but adjacent area of target. To describe this general method of eliminating the cross-dilution effect, we shall use the term "co-analysis".
The method of co-analysis previously disclosed is one which is readily applicable to apparatus of relatively large scale, with a rotating target, such as is appropriate for therapy. In apparatus of small scale, however, such as is required for use down hole for well-logging, the use of a rotating target is impractical. As a practical matter, the target must be fixed and compact, preferably in the form of a disc or cone, not exceeding 1.5 inches in diameter. If a magnetic field of fixed direction is used to co-analyse an ion beam with such a target, the ions intersect the target in a narrow band, leaving most of the target unused, with all of the ion beam concentrated on a relatively small fraction of the area of the target, thereby heating it to correspondingly high temperature and shortening its life.